Chemical insights into the formation of Cu2ZnSnS4 films from all-aqueous dispersions for low-cost solar cells

Abstract

Cu₂ZnSnS₄ (CZTS) shows great potential for photovoltaic application because of its non-toxic earth-abundant components and good optoelectronic properties. Combining low-cost and environmentally friendly routes would be the most favorable approach for the development of CZTS solar cells. In this context, development of Cu₂ZnSnS₄ (CZTS) films from all-aqueous CZTS nanocrystals inks represents an interesting challenge. Here, we have highlighted a condensation regulation by the alkali ion size observed in the alkali series Li⁺ < Na⁺ < K⁺ < Rb⁺ < Cs⁺, and demonstrated the chemical stability of Cu₂ZnSnS₄ surfaces in basic aqueous dispersions. Data such as optimal nanocrystal size, critical cracking thickness and average thickness to fabricate micron crack-free films from all-aqueous chalcogenide nanocrystals dispersions were determined. From these results, a proof of concept for the formation of a crack-free film of 2.2 μm formed from an all-aqueous CZTS nanocrystals ink is given. When employing low-cost materials, removal of carbon impurities represents another important challenge. With the objective to fabricate residue-free films, a specific annealing strategy is proposed involving a high temperature purification step under Se partial pressure. Carbon removal is thus achieved via the CSe₂ gas formation, simultaneously to the amorphous domains crystallization as demonstrated by Raman spectroscopy. These source data favoring the formation of residue-free, crack-free, annealed films should assist the large scale development of CZTS solar cells from low-cost and environmentally friendly, all -aqueous inks.